1. Field of the Invention
The present invention relates to processing of semiconductor materials. More particularly, the present invention relates to processing of indium antimonide.
2. Description of The Prior Art and Related Information
Various semiconductor compounds having existing or emerging commercial applications are not suitable for standard silicon processing technologies. In particular, indium antimonide (InSb) has increasing commercial interest but is unsuited for conventional semiconductor processing. For example, one application of InSb is for use in detection of infrared radiation. In such applications, large arrays of InSb photodiode integrated circuits are commonly employed and each InSb photodiode integrated circuit in turn contains an array of many individual diodes. Due to the high performance characteristics required for most commercial applications of InSb infrared detectors, constraints are placed on the InSb photodiodes that are difficult to simultaneously realize. Specifically, the individual photodiode junctions must provide both high performance and uniformity. InSb infrared detectors having the highest performance characteristics are fabricated by the thermal diffusion process; i.e., the dopant used to create the photodiode junction is diffused into the InSb crystal structure. This process minimizes damage to the crystal structure and provides good performance characteristics. On the other hand, the InSb detectors having the most uniform diode arrays are fabricated by ion implanting the dopant. The ion implantation step causes damage to the InSb crystal structure, however, thereby degrading the performance characteristics of the diode. Accordingly, to maximize both performance and uniformity of the InSb photodiode array, either the uniformity of the diodes formed by the diffusion process must be enhanced or the performance of the diodes formed by the ion implant process must be enhanced.
To enhance the performance of ion implanted InSb photodiode arrays, an annealing process has been employed that improves the performance of the ion implanted InSb photodiodes by repairing the damage to the crystal structure caused by the ion implantation. Due to the relatively high vapor pressure of InSb, however, the ion implanted InSb photodiode junctions cannot be annealed in a typical annealing step such as is normally employed for silicon integrated circuit processing. If such an annealing step were performed in vacuum, or in any other open environment such as in silicon integrated circuit processing, the surface of the InSb photodiode would be seriously pitted by the tendency of the elemental indium and antimony to evaporate out of the crystal. Accordingly, prior art methods for annealing InSb photodiode arrays have employed closed capsule annealing methods to prevent evaporation of the elemental indium and antimonide from the crystal structure during the annealing process.
In FIG. 1, a prior art closed ampoule annealing method for curing ion implantation damage in InSb photodiode arrays is illustrated. Referring to FIG. 1(a) a quartz ampoule 10 is isometrically illustrated with two InSb wafers 12, 14 shown resting on a wafer support tray 16 mounted horizontally within the ampoule 10. Also positioned on the wafer support tray 16 is a quartz cup 18 for holding elemental antimony which will be vaporized during the annealing process and provide a background partial pressure of antimony in the ampoule. A second quartz cup (not shown) for holding elemental indium may also be employed. The amount of elemental antimony in quartz cup 18 is chosen so as to provide the desired partial pressure of antimony at the annealing temperature. In practice, however, the actual amount of elemental antimony employed provides only approximately the desired partial pressure at the wafer surface due to temperature variations along the capsule as a function of heating and cooling time. Therefore, since the actual partial pressure in the ampoule during annealing only approximates the desired amount, some thermal micropitting does occur. Also shown in FIG. 1(a) is an evacuation tube 20 attached to one end 21 of ampoule 10. As shown, ampoule end 21 is initially separated from the remainder of ampoule 10 to allow for loading of the wafers 12 and 14 and elemental antimony. As shown in FIG. 1(b), once the InSb wafers 12 and 14 and antimony cup 18 are inserted into the ampoule 10 the two parts of ampoule 10 are sealed at a butt seal 22. Ampoule 10 is chosen sufficiently long such that the heat generated during sealing along butt seal 22 will not result in heating of wafers 12 and 14. After sealing ampoule 10 is evacuated through evacuation tube 20. In FIG. 1(c) ampoule 10 is illustrated with the vaccuum tube 20 removed and the opening sealed, ready for insertion of the ampoule 10 into a furnace for annealing. The ampoule 10 is placed in a furnace at an elevated temperature, in the range 200.degree. C.-475.degree. C., for times of the order of 15 minutes to 2 hours. After the prescribed annealing time, the ampoule is quenched, i.e. rapidly cooled to room temperature. After quenching the ampoule 10 is broken open as illustrated in FIG. 1(d), to remove the annealed InSb wafers 12 and 14.
The ampoule annealing method described above suffers from a number of disadvantages. First of all, although the elevated temperature in the closed ampoule can supply the energy necessary to heal dislocation damage in the crystal structure of the InSb wafer caused by the implant process, the environment at the surface of the wafer is not ideal. This allows thermal micropitting to occur, which limits the performance of the ion implanted photodiode. To minimize this thermal micropitting the closed ampoule anneal is limited to times that are too short and temperatures that are too low to completely remove the damage to the crystal from the ion implanting step. Thus, the closed ampoule process not only results in new damage to the implanted layer, but also does not completely remove the damage that the annealing process is meant to eliminate. Such new and residual crystal structure damage is the reason why prior art ion implanted photodiode InSb arrays have reduced performance characteristics relative to diffusion fabricated arrays.
Secondly, the vaporized indium and/or antimony in the ampoule will unavoidably be deposited on the wafer surface by condensation during the quenching process. This unwanted deposition on the wafer surface can cause loss of the desired diode performance characteristics and can also cause the wafers to be visually rejected for subsequent fabrication steps.
Additionally, the closed quartz ampoule annealing process is commercially undesirable. The process is both expensive and unsuited for large scale commercial production. Each time an anneal is to be performed a quartz ampoule needs to be fabricated in two parts. The two parts then need to be carefully cleaned, assembled around the wafers, evacuated, placed in a furnace, annealed, quenched, and broken open. The ampoule assembly and cleaning steps are time consuming and require highly skilled personnel. While the time involved in actually annealing at elevated temperature is only approximately an hour, the other preparation steps typically require the best part of a day. Therefore, large scale production of high quality ion implanted InSb photodiode wafer arrays by the closed ampoule annealing method would be prohibitively time consuming and expensive. Without annealing, however, the relatively poor performance characteristics of ion implanted photodiode arrays requires compensation by additional cryogenic cooling to relatively lower temperatures. The extra cost of such additional cryogenic equipment may amount to millions of dollars in system costs.
Accordingly, a need presently exists for improving the performance characteristics of ion implanted photodiode arrays in a commercially viable manner.